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Displacement hull

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Displacement hull


A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.

The structure of the hull varies depending on the vessel type. In a typical modern steel ship, the structure consists of watertight and non-tight decks, major transverse and watertight (and also sometimes non-tight or longitudinal) members called bulkheads, intermediate members such as girders, stringers and webs, and minor members called ordinary transverse frames, frames, or longitudinals, depending on the structural arrangement. The uppermost continuous deck may be called the "upper deck", "weather deck", "spar deck", "main deck", or simply "deck". The particular name given depends on the context—the type of ship or boat, the arrangement, or even where it sails. Not all hulls are decked (for instance a dinghy).

In a typical wooden sailboat, the hull is constructed of wooden planking, supported by transverse frames (often referred to as ribs) and bulkheads, which are further tied together by longitudinal stringers or ceiling. Often but not always there is a centerline longitudinal member called a keel. In fiberglass or composite hulls, the structure may resemble wooden or steel vessels to some extent, or be of a monocoque arrangement. In many cases, composite hulls are built by sandwiching thin fiber-reinforced skins over a lightweight but reasonably rigid core of foam, balsa wood, impregnated paper honeycomb or other material.


General features

The shape of the hull is entirely dependent upon the needs of the design. Shapes range from a nearly perfect box in the case of scow barges, to a needle-sharp surface of revolution in the case of a racing multihull sailboat. The shape is chosen to strike a balance between cost, hydrostatic considerations (accommodation, load carrying and stability), hydrodynamics (speed, power requirements, and motion and behavior in a seaway) and special considerations for the ship's role, such as the rounded bow of an icebreaker or the flat bottom of a landing craft.

Hull shapes

Template:See Hulls come in many varieties and can have composite shape, (e.g., a fine entry forward and inverted bell shape aft), but are grouped primarily as follows:

  • Moulded, round bilged or soft-chined. Examples are the round bilge, semi-round bilge and s-bottom hull.
defined as smooth curves
  • Chined and Hard-chined. Examples are the flat-bottom (chined), v-bottom and multi-bottom hull (hard chined).
have at least one pronounced knuckle throughout all or most of their length

Categorisation

After this they can be categorized as:

  • Displacement
the hull is supported exclusively or predominantly by buoyancy. Vessels that have this type of hull travel through the water at a limited rate which is defined by the waterline length. They are often heavier than planing types, though not always.
  • Semi-displacement, or semi-planing
the hull form is capable of developing a moderate amount of dynamic lift, however, most of the vessel's weight is still supported through buoyancy
  • Planing
the planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. The dynamic lift reduces the wetted surface and therefore also the drag. They are sometimes flat-bottomed, sometimes V-bottomed and sometimes round-bilged. The most common form is to have at least one chine, which makes for more efficient planing and can throw spray down. Planing hulls are more efficient at higher speeds, although they still require more energy to achieve these speeds. (see: Planing (sailing), Hull speed).

Most used hull forms

At present, the most widely used form is the round bilge hull.[1]

The inverted bell shape of the hull, with smaller payload the waterline cross-section is less, hence the resistance is less and the speed is higher. With higher payload the outward bend provides smoother performance in waves. As such, the inverted bell shape is a popular form used with planing hulls.

Hull forms

Smooth curve hulls

Template:See

Smooth curve hulls are hulls which use, just like the curved hulls, a sword or an attached keel.

Semi round bilge hulls are somewhat less round. The advantage of the semi-round is that it is a nice middle between the S-bottom and chined hull. Typical examples of a semi-round bilge hull can be found in the Centaur and Laser cruising dinghies.


S-bottom hulls are hulls shaped like an s . In the s-bottom, the hull runs smooth to the keel. As there are no sharp corners in the fuselage. Boats with this hull have a fixed keel, or a kielmidzwaard. This is a short fixed keel, with a swing keel inside. Examples of cruising dinghies that use this s-shape are the yngling and Randmeer.

Chined and hard-chined hulls

Template:See

A chined hull consists of straight, smooth, tall, long, or short plates, which are set at an angle to each other. The chined hull is the most simple hull shape because it works with only straight planks. These boards are often bent lengthwise. Most home-made constructed boats are chined hull boats. The Cajun "pirogue" is an example of a craft with hard chines. Benefits of this type of boating activity is the low production cost and the (usually) fairly flat bottom, making the boat faster at planing. Chined hulls can also make use of a daggerboard or attached keel.

Chined hulls can be divided up into 3 shapes:

  • V-bottom chined hulls
  • Flat-bottom chined hulls
  • Multi-chined hulls.

Appendages

  • A protrusion below the waterline forward is called a bulbous bow and is fitted on some hulls to reduce the wave making resistance drag and thus increase fuel efficiency. Bulbs fitted at the stern are less common but accomplish a similar task. (see also: Naval architecture)
  • A keel may be fitted on a hull to increase the transverse stability, directional stability or to create lift.
  • Control devices such as a rudder, trim tabs or stabilizing fins may be fitted.

Terms

  • Bow is the front part of the hull
  • Stern is the rear part of the hull
  • Port is the left side of the vessel when facing the bow
  • Starboard is the right side of the vessel when facing the bow
  • Waterline is an imaginary line circumscribing the hull that matches the surface of the water when the hull is not moving.
  • Amidships is the middle portion of the vessel in the fore and aft direction.
  • Baseline is an imaginary reference line used to measure vertical distances from.

Metrics


Hull forms are defined as follows:

  • Block measures that define the principal dimensions. They are:
  • Length overall (LOA) is the extreme length from one end to the other.
  • Length at the waterline (LWL) is the length from the forwardmost point of the waterline measured in profile to the stern-most point of the waterline.
  • Length between perpendiculars (LBP or LPP) is the length of the summer load waterline from the stern post to the point where it crosses the stem. (see also p/p)
  • Beam or breadth (B) is the width of the hull. (ex: BWL is the maximum beam at the waterline)
  • Moulded depth (D) is the vertical distance measured from the top of the keel to the underside of the upper deck at side.[2]
  • Draft (d) or (T) is the vertical distance from the bottom of the keel to the waterline.
  • Freeboard (FB) is depth plus the height of the keel structure minus draft.
  • Form derivatives that are calculated from the shape and the block measures. They are:
  • Volume (V or ) is the volume of water displaced by the hull.
  • Displacement (Δ) is the weight of water equivalent to the immersed volume of the hull.
  • Longitudinal centre of buoyancy (LCB) is the longitudinal distance from a point of reference (often midships) to the centre of the displaced volume of water when the hull is not moving. Note that the longitudinal centre of gravity or centre of the weight of the vessel must align with the LCB when the hull is in equilibrium.
  • Vertical centre of buoyancy (VCB) is the vertical distance from a point of reference (often the baseline) to the centre of the displaced volume of water when the hull is not moving.
  • Longitudinal centre of floatation (LCF) is the longitudinal distance from a point of reference (often midships) to the centre of the area of waterplane when the hull is not moving. This can be visualized as being the area defined by the water's surface and the hull.
  • Coefficients[3] help compare hull forms as well:
1) Block coefficient (Cb) is the volume (V) divided by the LWL x BWL x T. If you draw a box around the submerged part of the ship, it is the ratio of the box volume occupied by the ship. It gives a sense of how much of the block defined by the LWL, beam (B) & draft (T) is filled by the hull. Full forms such as oil tankers will have a high Cb where fine shapes such as sailboats will have a low Cb.

C_b = \frac {V}{L_{WL} \cdot B \cdot T}

2) Midship coefficient (Cm or Cx) is the cross-sectional area (Ax) of the slice at midships (or at the largest section for Cx) divided by beam x draft. It displays the ratio of the largest underwater section of the hull to a rectangle of the same overall width and depth as the underwater section of the hull. This defines the fullness of the underbody. A low Cm indicates a cut-away mid-section and a high Cm indicates a boxy section shape. Sailboats have a cut-away mid-section with low Cx whereas cargo vessels have a boxy section with high Cx to help increase the Cb.

C_m = \frac {A_m}{B \cdot T}

3) Prismatic coefficient (Cp) is the volume (V) divided by Lpp x Ax. It displays the ratio of the immersed volume of the hull to a volume of a prism with equal length to the ship and cross-sectional area equal to the largest underwater section of the hull (midship section). This is used to evaluate the distribution of the volume of the underbody. A low or fine Cp indicates a full mid-section and fine ends, a high or full Cp indicates a boat with fuller ends. Planing hulls and other highspeed hulls tend towards a higher Cp. Efficient displacement hulls travelling at a low Froude number will tend to have a low Cp.

C_p = \frac {V}{L_{pp} \cdot A_m}

4) Waterplane coefficient (Cw) is the waterplane area divided by Lpp x B. The waterplane coefficient expresses the fullness of the waterplane, or the ratio of the waterplane area to a rectangle of the same length and width. A low Cw figure indicates fine ends and a high Cw figure indicates fuller ends. High Cw improves stability as well as handling behavior in rough conditions.

C_w = \frac {A_w}{L_{pp} \cdot B}

Note:

C_b = {C_{p} \cdot C_{m} }

History

Rafts have a hull of sorts, however, hulls of the earliest design are thought to have each consisted of a hollowed out tree bole: in effect the first canoes. Hull form then proceeded to the coracle shape and on to more sophisticated forms as the science of naval architecture advanced.

See also

Notes

References

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